Today, batteries are used to provide power to many devices. These devices are more often portable or mobile devices. More recently, batteries have been a primary power source used for laptop and notebook computers, cellular phones, hand-held/mobile video recorders and many other electronic devices. Current battery cells include nickel cadmium, nickel metal hydride, lithium-ion, alkaline, acid, zinc, etc.
Typically, a group of batteries are strung together and encased in a plastic-molded pack. The cells in a pack are strung together by electrically coupling the positive terminal of one battery to the negative terminal of the next battery, with the negative terminal of the first battery in the string being the negative contact of the battery pack and the positive terminal of the last battery in the string being the positive contact of the battery pack. The plastic battery pack may also contain a logic board (e.g., PCB) that contains a integrated circuit (IC), an analog-to-digital (A/D) converter and/or a serial communication device to monitor the state of the battery pack and provide access to the state by a unit external to the battery pack. However, this logic board does not monitor each cell independently, only the group of battery cells as a whole. A temperature sensor may also be included in the battery pack to determine the temperature within the battery pack. The temperature reading obtained using the temperature sensor is output from the battery pack along with the positive and negative contacts of the first and last batteries in the string respectively.
One problem with current battery packs of the prior art is their expense. Specifically, approximately 20% of the cost of a battery pack is due to the molded plastic used to encase the battery cells. The plastic is normally custom molded for use in a specific application, such that there is no interchangeability between the various battery packs. For example, a battery pack designed for use in a specific laptop or notebook computer, normally cannot be used in other computers. This is due in part to the necessary functionality that is included within the battery packs to monitor and control the batteries as a whole. It is desirable to add some interchangeability to the battery packs. By integrating monitoring and control functionality into the battery cells themselves, the circuit board within a battery pack may be eliminated, thereby leaving only the battery cells in the battery pack. With cells of a particular type being approximately the same size, the battery packs can be the same size and interchangeable for different applications.
Another problem with the battery packs of the prior art is that they do not provide charging of individual cells. The entire string of batteries in the battery pack is coupled up to a charger in order to charge the battery pack as a whole. Cells within the inner portion of a string of batteries are not monitored individually, such that there is no method of guaranteeing that these inner cells are fully changed without always overcharging.
The battery packs of the prior art do not provide for monitoring of individual cells. Because individual cells are not monitored, the user has no way of knowing the current charge level of individual cells in the pack or that a battery cell in the pack is damaged. Charging a damaged cell may be dangerous because the cell may vent, become very hot, do nothing, or explode. Thus, without the ability to monitor, the user may be put in danger when attempting to charge the battery pack. Moreover, without monitoring, difficulty exists in maintaining a cell's charging within its optimum operating range. For instance, certain cells, such as lithium-ion cells, operate when they are charged to a particular range. If they are over-charged, they explode. If they are discharged too low, then they cannot be charged again. Because there is no monitoring in the prior art of individual cells, one cannot determine if one of the cells in the pack has become over-charged, such that it may explode, or has become discharged too low, such that it cannot be charged again. Therefore, it is desirable to be able to monitor individual battery cells in a battery pack.
Furthermore, because individual cells are not monitored, cells may become unbalanced. When cells become unbalanced, it is possible that a reverse cell voltage could occur. That is, as cells are normally coupled together with the positive terminal connected to the negative terminal of a subsequent battery cell in the string, if a cell becomes discharged too low, a cell reversal may occur. In cell reversal, if the charge in the cell becomes zero volts, the positive and negative designations lose their meaning and terminals of an individual battery cell may switch designations such that the positive terminal and the negative terminal of a zero volt battery cell switch to become negative and positive respectively. In such a situation, the other battery cells start to charge the zero volt cell and place energy into it due to its terminals being connected to the same terminals of its neighboring battery cells. In the prior art, this may occur unknown to the user since there is no monitoring of individual cells in the battery pack. Thus, it is desirable to provide monitoring of individual cells to avoid cell reversal.
The present invention provides a mechanism to monitor and charge each cell independently. In doing so, the present invention eliminates unbalanced cells, reversed cell voltages and provides a mechanism to determine if a cell is damaged to prevent damaged cells from being charged.